Above the pole of the magnetic bar is placed the iron vibrating plate L L, which is coated either with black or yellow varnish, with tin or blue oxide, but which must always be very thin. This plate is in the form of a disk, and by its rim, resting on a caoutchouc ring, it is firmly fixed to the circular edges of the wooden box, which is for this purpose made in two pieces. These pieces are adjusted to each other, either by screws or by spirals cut in half the thickness of the wood. This disk ought to be as near as possible to the polar end of the magnet, yet not so near as to produce contact between the two by the vibrations of the voice. Finally, the mouthpiece R R′ (fig. 21), which is in form of a wide funnel, terminates the upper part of the box, and should be so arranged as to leave a certain space between the disk and the edges of the hole V, which is open in its centre. The size of the box should be so calculated as to permit of its acting as a sounding-box, without however provoking echoes and a confusion of sounds. [Illustration: FIG. 22.] When the instrument is properly made, it will produce very marked effects; and M. Pollard, one of the first Frenchmen to take up the study of telephones, has written as follows on the subject: ‘The instrument which I have prepared gives results which are truly astonishing. In the first place, when considering the resistance, the introduction into the circuit of five or six persons does not sensibly diminish the intensity of sounds. On putting an instrument to each ear, the sensation is precisely the same as if the correspondent were speaking some yards behind. The intensity, the clearness, the purity of tone are irreproachable. ‘I can speak to my colleague in quite an undertone, scarcely breathing as I may say, and persons placed within two yards of me will be unable to catch a single word of our conversation. ‘On the part of the receiver, if anyone raises his voice to call me, I hear the call in all parts of my office, at least when silence prevails there; at any rate, when I am seated at my table with the instrument some yards off, I can always hear the call. In order to increase the intensity of sound, I fitted the mouthpiece with a copper horn of conical shape, and under these conditions words spoken in my bureau two or three yards from the mouthpiece can be heard at the other end of the line; from my station, a little more than a yard from the tube, I can hear and speak to my colleague without effort.’